Structure–Reactivity Correlations and Mechanistic Understanding of the Photorearrangement and Photosalient Effect of α‑Santonin and Its Derivatives in Solutions, Crystals, and Nanocrystalline Suspensions

The synthesis, crystal packing and
photochemical reactions of α-santonin
(1a) and its methyl, ethyl, n-propyl,
and n-butyl derivatives (1b–e) are described to explore the effect that a photochemically
benign yet structurally significant synthetic modification can have
on the solid-state photochemical reactivity. The structures of the
derivatives were determined using single crystal X-ray diffraction
and compared against the packing of α-santonin. A cage dimer
(12a) found exclusively upon irradiation of 1a in the solid state was not found when the other derivatives were
exposed to light, because the alkylation proved to perturb the crystal
packing away from an optimal dimerization alignment. Using a high-speed
camera, we monitored the photosalient effect of α-santonin and
found it to occur at an angle orthogonal to the b-axis of the unit cell, which we suspect is caused by the formation
of the cage dimer 12. The photochemistry of 1a–e in solution and crystalline suspensions was
also analyzed. The solution photochemistry was in accord with literature
precedence, and the crystalline suspensions yielded a variety of photoproducts
including a tertiary alcohol (7b–e), which is not commonly observed in neutral water. An exocyclic
alkene photoproduct (8b) was also discovered, and its
presence is hypothesized to be caused by an intermolecular deprotonation
caused by a water molecule present in the crystal.

本文报道了α-山道年（α-santonin，1a）及其甲基、乙基、正丙基与正丁基衍生物（1b–e）的合成、晶体堆积行为及光化学反应，旨在探究一种光化学惰性但结构意义显著的合成修饰对固态光化学反应活性的影响。通过单晶X射线衍射（single crystal X-ray diffraction）测定了各衍生物的晶体结构，并与α-山道年的晶体堆积模式进行对比。仅在固态下辐照1a时得到的笼状二聚体（cage dimer，12a），在其余衍生物的光照实验中均未获得，这是因为烷基化修饰破坏了晶体堆积中最优的二聚体排列取向。借助高速相机，我们观测到α-山道年的光致动效应（photosalient effect），其发生方向与晶胞的b轴正交，我们推测该现象源于笼状二聚体12的生成。此外，本文还分析了1a–e在溶液及晶体悬浮液中的光化学反应：溶液中的光化学反应结果与已有文献报道一致，而晶体悬浮液则生成了多种光产物，包括一种在中性水中较为罕见的叔醇（7b–e）。我们还发现了一种外环烯烃（exocyclic alkene）类光产物（8b），推测其生成源于晶体晶格中存在的水分子引发的分子间脱质子反应。